Inconsistent behavior when using additionalParameters

## Issue

If I specify an additional parameter via `additionalParameters` on `iOS`, it is not passed to the `tokenEndpoint`.
However, for `android` the additional parameter is used for both the `authorizationEndpoint` and the `tokenEndpoint`.

I would expect the behavior to be identical on both platforms.
Furthermore, it would be great if additional parameters could be defined for both requests.

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## Environment

* **Your Identity Provider**: `pCon.login`
* **Platform that you're experiencing the issue on**: `iOS`
* **Are you using Expo?**: `no`